Extraction - Separation Process From Process Engineering

What is an extraction?

Extraction is a physical or chemical separation process in which one or more components are dissolved out of a mixture of substances using an extraction agent (solid, liquid or gaseous). The mixture of substances is referred to as the extraction material, while the dissolved components, even if they are still in solution, are known as the extract.

If no chemical change to the substance takes place during extraction (e.g. only dissolution or adsorption), this is referred to as a physical process. If, on the other hand, the substance is chemically transformed, it is referred to as a chemical extraction.

Properties of extraction:

• Extraction utilizes different solubilities of the components in the substance mixture.
• The extraction agent used pulls the more soluble substance out of the mixture.
• The process differs from other separation methods such as filtration, which are based on physical properties such as particle size.

Extraction is an essential component in the separation of substances and the preparation of samples for chemical analysis. Thorough plant design forms the basis for efficient implementation on an industrial scale.

What extraction methods are there?

The selection of a suitable process often starts with pre-engineering, where feasibility and initial concepts are developed.

Classic extraction methods

Solid-liquid extraction

In solid-liquid extraction, substances are extracted from a solid material using a liquid solvent. Examples include coffee preparation or petroleum production with supercritical CO₂. Variants of this procedure include:

• Maceration: Soaking solids in solvents without additional energy input.
• Digest: Maceration with heat and agitation to speed up extraction.
• Leaching/washing: Removing substances such as minerals or pollutants from solids by rinsing.

Liquid-liquid extraction

In this process, solutes are extracted from a liquid with another liquid that has a different affinity for the target substances. Examples include:

• Shake out: Use in the laboratory to separate substances using different densities and solubilities.
• Liquid membrane permeation: Use of emulsions to selectively transport substances.

Gas-liquid extraction

Here, a gas is absorbed by a liquid in order to remove specific components. Examples include gas scrubbers to remove carbon dioxide and applications in gas chromatography.

Liquid-gas extraction

A gas extracts substances from a liquid, for example by stripping or gas chromatographic processes. This is often used to purify and enrich substances.

Liquid-solid extraction

This process uses adsorption to enrich substances from liquids onto solids. A typical example is the removal of chlorinated hydrocarbons from drinking water using activated carbon.

Extraction with specific aggregate states

Specialized methods use different phase transitions or aggregate states, such as:

• Steam distillation: Highly volatile components are carried away by steam.
• Adsorption or absorption: Removal of gases such as water vapor through solids (e.g. with calcium oxide).
• Enfleurage: Extraction of essential oils in the perfume industry by fat absorption.

Modern and specialized extraction processes

Supercritical fluid extraction (SFE)

A supercritical fluid, usually CO2, serves as a solvent. This process enables precise solubility control by adjusting pressure and temperature. It is ideal for temperature-sensitive substances such as flavors or active ingredients.

Microwave-assisted extraction (MAE)

Here, microwaves are used to directly heat the solvent and the sample. The increased temperature and pressure promote rapid and efficient extraction.

Ultrasound-assisted extraction (UAE)

Ultrasonic waves generate cavitation, which causes cell walls to break open and accelerate the diffusion of target substances into the solvent. This process is particularly energy-efficient and gentle.

Soxhlet extraction

In Soxhlet extraction, the solvent is continuously heated, evaporated and condensed, causing it to circulate through the sample in a closed circuit. It is particularly efficient for poorly soluble substances.

Liquid-phase microextraction (LPME)

LPME minimizes the use of solvents by using minute amounts for extraction into microvolumes. The technology is cost-effective and suitable for sensitive analyses.

High Pressure Solid Extraction (PLE)

Under increased pressure, solvents are more effectively pressed into the sample matrix, which enables the extraction of even substances that are difficult to access.

Ion exchange extraction

This process uses ion exchange resins to specifically isolate charged molecules from liquids. It is particularly selective and is often used in chemical and biotechnology research.

Membrane extraction

Membranes serve here as a semi-permeable barrier that selectively allows specific molecules to pass through. It is ideal for environmentally friendly and highly specific applications.

Carrying out the extraction through dissolution processes

Carrying out the extraction requires careful planning of the plant components, which are developed as part of basic and detailed engineering. Extraction through dissolution processes takes place in several defined steps, which are based on the different solubility of the components in the mixture. The aim is to isolate the desired substance (extract) from a mixture of substances. Chemical and physical principles of mass transfer come into play here.

Extraction steps:

1. Mixing the mixture with the solvent:
   • In order to maximize mass transfer, the substance mixture is intensively mixed with the extractant. This creates as large a contact area as possible.
   • In the laboratory, this is often done in equipment such as a Soxhlet extractor or a shaking hopper. Closed mixers or mixer-settlers are used on an industrial scale.
2. Solution process and equilibrium setting:
   • The desired substance is partially dissolved in the extractant. The system achieves a dynamic equilibrium in which the substance is distributed between the two phases.
   • The process follows Nernst's distribution law, which describes the distribution of substances between two immiscible phases.
3. Separation of phases:
   • After the dissolution process, the extractant is separated from the remaining phase. For this, it is necessary that the two phases do not mix (mixing gap).
   • Techniques such as decanting, centrifuging or the use of shaking hoppers and separators enable phase separation. A sufficient density difference between the phases is crucial.
4. Extraction agent preparation:
   • The extractant is treated to recover the isolated substance. This is often done by distillation, for example rectification.
   • The aim is to preserve the pure recyclable material and make the solvent available again for a further extraction process.

On a technical scale and in precise plant construction, extraction is often carried out in several stages, using columns or centrifugal extractors to make the separation process efficient.

Solvent requirements

For successful extraction, the solvent must meet certain properties that influence the separation process and efficiency.

Selectivity and solubility

• The solvent should only dissolve the extract as specifically as possible and leave other components of the mixture largely unaffected. Since this is often not completely successful in practice, downstream separation processes must remove unwanted substances.
• A high solubility of the extract in the solvent speeds up the process and allows a higher substance concentration.

Chemical properties

• Inertness: The solvent must not enter into a chemical reaction with the extract.
• Polar differences: There should be a sufficient polarity difference between the solvent and the extraction material to ensure good phase separation.

Physical properties

• Density difference: In liquid-liquid extractions, a sufficient difference in density facilitates phase separation.
• Low boiling point: A low-boiling solvent reduces the energy required for recovery and makes the process more economical.
• Low viscosity: This promotes rapid diffusion of the substances.

Safety and environmental sustainability

• The solvent should not be flammable, toxic or corrosive. Environmental hazards caused by the solvent must be minimized.
• It should be possible to dispose of or recover the solvent economically and sustainably.

These requirements ensure that the extraction can be carried out efficiently, safely and in an environmentally friendly manner without affecting the profitability of the process.

Which parameters influence extraction?

1. Surface area of the extraction material

• A large surface area of the extraction material is essential, since the amount of extractable substances is proportional to the contact area between the extraction material and the solvent.
• Solids extraction: Crushing the extraction material into a fine powder increases the surface area.
• Liquid-liquid extraction: Intensive stirring or emulsification produce small liquid droplets that offer a larger exchange area.

2. Difference in concentration between extract and solvent

• A large concentration gradient is the driving force of extraction.
• Methods to obtain a high gradient:
  • Frequent replacement of the loaded solvent with fresh one.
  • Quick removal of the dissolved extract from the surface of the extraction material.

3. Diffusion resistance

• The diffusion resistance makes extraction difficult and depends on the particle size and the porosity of the extraction material.
• Measures to reduce diffusion resistance:
  • Use of fine-pored or heavily shredded materials.
  • Optimizing the permeability of the solvent.

4. Temperature

• Higher temperatures promote extraction by:
  • Increased molecular mobility (heat movement).
  • Lower solvent viscosity, which makes it easier to penetrate and dissolve the target substances.
• Note: The temperature should only be raised if the stability of the target substances is ensured.

5. Solubility

• The solubility of the target substances in the solvent is a key factor.
• Solvents should be selected in such a way that the target substances can be optimally absorbed.

6. Penetrating behavior of the solvent

• The solvent must be able to penetrate easily into the extraction material in order to effectively dissolve the target substances.
• Optimum wettability of the extraction material is crucial.

7. Interfering factors in the sample

• In complex samples, contaminants can reduce extraction efficiency.
• Measures to reduce disruptions:
  • Use of suitable phases to separate interfering compounds.
  • Use of various solvents with varying polarity to better separate fractions.

Extraction applications

Extraction is used in various industries.

Chemical industry

Organic-chemical industry

Extraction is preferred when distillation or rectification is impracticable or economical. It is particularly useful for:

• Heat-sensitive substances: Substances such as antibiotics or natural substances that break down before the boiling temperature is reached.
• Azeotropic mixtures: BTX aromatics are thus extracted efficiently without using complex rectification methods.
• Similar boiling point of components: Extraction is often the better choice when there are small temperature differences.
• Heavy/low boiler separations: Small amounts of a low-volatile substance can thus be removed from highly volatile substances.

Inorganic-chemical industry

Extraction is essential in inorganic chemistry:

• Titanium dioxide recovery: An essential process for producing this important pigment.
• Bauxite processing: Aluminum is obtained through chemical reactions and extraction of unwanted constituents such as iron hydroxide.
• Reactor fuel element processing: Extraction is used here to recover valuable substances.

Pharmaceutical industry

Pharmaceuticals use extraction to obtain active ingredients from medicinal plants and use them in pharmaceuticals. advantages:

• Standardization: Safe quality through defined quantities of active ingredients. Comprehensive documentation ensures traceability and meets regulatory requirements.
• Improved compatibility: removal of unwanted by-products.
• Efficient drug enrichment: Reducing the amount of substance required. Common processes include maceration, Soxhlet processes, and countercurrent extraction.

Food industry

Extraction is central to food production:

• Cooking oil production: Extraction by pressing and hexane extraction.
• Decaffeination: Supercritical CO2 gently removes caffeine from coffee.
• Aroma extraction: Extraction of dyes, aromas and other valuable substances.
• Impregnation: Supercritical CO₂ is used to treat wood and other materials.

Environmental technology

Wastewater treatment

Extraction is used in sewage treatment plants to remove substances such as phosphates. These can be reused as fertilisers.

Soil remediation

Contaminated soils can be freed from pollutants through high-pressure extraction with compressed gases such as CO₂.

Petrochemical industry

Extraction helps separate aromatics, hydrocarbons, and other valuable components from crude oil. It is a key technology for refining petrochemical products.

Energy industry

In nuclear energy, extraction is used to reprocess fuel elements to recover fissile material and minimize waste volumes.

Biotechnology

Biotechnology uses extraction processes to separate sensitive molecules such as proteins:

• Aqueous two-phase systems: For example, polyethylene glycol-dextran systems, which enable gentle separation.

Materials science

Extraction is used to produce high-performance materials and to recover specific substances. It also plays a role in the coloring and impregnation of fabrics.

Perfume and cosmetics industry

Fragrances such as rose or jasmine oil are obtained using extraction methods such as enfleurage. These processes ensure the purity and quality of the sensitive substances.

Everyday life

Extraction is also present in the home:

• Washing textiles and dishes: Dirt is efficiently removed with water and cleaning agents.
• Coffee preparation: Water extraction provides soluble coffee in the form of powder or freeze-dried granules.

FAQ about extraction

What is extraction?

Extraction is a chemical separation process in which a solvent is used to selectively remove substances from a mixture. It is used to separate, purify or enrich substances, e.g. to remove interfering substances or to improve detection accuracy in analysis.

How does extraction work simply explained?

During extraction, you add an extractant to a mixture of substances. This compound dissolves certain ingredients while leaving others behind. The principle is based on solubility: Some substances dissolve better in the extractant than others. After extraction, you can separate the extractant with the dissolved substances from the remaining components of the mixture, for example by decanting, filtering or separating.

How do you do an extraction?

1. Preparation: Choose a suitable extractant that specifically dissolves the desired substances.
2. Procedure: Mix the substance mixture with the extractant. Shake or stir the mixture so that the mass transfer takes place.
3. Separation: Remove the extractant that contains the solutes from the rest of the mixture.
4. Further processing: Concentrate or purify the extracted substance, if necessary, through further processes such as distillation or evaporation.

What is the difference between sample separation and sample extraction?

• Sample separation refers to all processes that separate mixtures of substances into their components. This includes methods such as filtration, centrifugation or distillation, which utilize various physical properties.
• Sample extraction is a specific form of sample separation. Here, a substance is specifically dissolved by an extraction agent based on its solubility. Extraction can also be used to enrich an analyte, which is rarely the case with general sample separation methods.

Extraction stands out due to the targeted use of solubility, while sample separation often exploits other physical properties such as density or boiling point.